1. Field of the Invention
The present invention relates to a pressure device, such as a pressing machine used, for example, in sheet metal working and, more particularly, to a pressure device which is capable of pressing operation requiring accurate position control and which, at the same time, involves a large pressing force and yet small driving energy.
2. Description of the Related Art
In a conventional type of press working machine, a hydraulic cylinder is widely used as a means of driving a ram that comes in contact with a workpiece and, in particular, an oil hydraulic cylinder is frequently used. In this type of hydraulic cylinder-operated pressing machine, it is necessary to perform press working as shown in FIG. 6, that is, working is conducted with the distance between a ram and a table kept constant.
FIG. 6 is an explanatory drawing of conventional working. In FIG. 6, numeral 31 indicates a table. With respect to this table 31, a ram 32 of a pressing machine moves up and down by use of an oil hydraulic cylinder, for example, to perform the press working of a workpiece 33. In order to accurately work the workpiece 33 to a thickness dimension t with this arrangement, the bottom end of the ram 32 is provided with projections 35 that have a height equal to the workpiece thickness t and protrude downward from a working surface 34.
With this arrangement, when the ram 32 is operated downward, the working surface 34 can perform prescribed working of the workpiece 33. Keeping the projections 35 of the ram 32 abutting against the table 31 allows the thickness dimension t of the workpiece 33 to be accurately maintained, free from dimensional variations. Thus, the working accuracy of the workpiece 33 can be improved.
The working method shown in FIG. 6, however, poses the following problem although the working accuracy can be improved by this method. That is, impact noises are inevitably generated because the ram 23 hits against the workpiece 33 in an impacting manner and because the projections 35 of the ram 32 also violently hit against the table 31. In particular, in the case of high-speed working where the working frequency of the ram 32 per unit time is high, noises become so great that they impair the working environment.
On the other hand, working by use of an electrically-driven press has so far been practiced, and it is known that this working method is favorable for preventing the generation of noises caused by the working as shown in FIG. 6 by the above hydraulic press, etc.
FIG. 7 is a longitudinal sectional view of essential portions of an example of the conventional electrically-driven press. This drawing is contained in Japanese Published Unexamined Patent Application No. Hei-6(1994)-218591, for example. In FIG. 7, reference numeral 41 indicates a pressing force generating means. The pressing force generating means 41 is housed within a head frame 44 installed on a column 43, which is formed integrally with a table 42.
Numeral 45 indicates a tubular body. The tubular body 45 is installed within the head frame 44 and is provided with a bearing part 46 at the top end thereof. Numeral 47 indicates a screw shaft. The top end of the screw shaft 47 is supported by the bearing part 46 in a suspended state. Numeral 48 indicates a ram shaft, which is formed in a hollow cylindrical shape. A nut 49, which engages with the screw shaft 47, is fixed to the top end of the ram shaft 48. The ram shaft 48 is installed so that it can move vertically within the tubular body 45. Numeral 50 indicates a pressing element detachably installed in the bottom end portion of the ram shaft 48. The screw shaft 47 and nut 49 are in ball-screw engagement.
Next, numeral 51 indicates a sliding guide post. The sliding guide post 51 comprises a guide portion 52 installed within the head frame 44, a sliding rod 53, and a connecting plate 54 installed between the ram shaft 48 and the bottom end of the sliding rod 53. Numeral 55 indicates a drive motor. The drive motor 55 is installed within the head frame 44 and drives the screw shaft 47 in both forward and reverse directions via a pulley 56 and a belt 57, which are installed in the top end portion of the screw shaft 47.
Note that measuring means, central processing unit, etc., which are not shown in the drawing, set the start and stop positions of the pressing element 50 and the rotational speed of the drive motor 55, give the drive motor 55 instructions for rotation in the forward and reverse directions, etc.
With the above construction of the electrically-operated press, as the screw shaft 47 is rotated by the operation of the drive motor 55 via the belt 57 and the pulley 56, the ram shaft 48 having the nut 49 fixed to the upper end thereof descends and the pressing element 50 abuts against a workpiece W with a pressing force in a preset position as shown by chain lines to perform the prescribed press working. After the completion of press working, the ram shaft 48 and pressing element 50 ascend by the reverse rotation of the drive motor 55 and return to the initial positions. By repeating the above operation, the prescribed press working can be accomplished on a plurality of workpieces W.
When an electrically-driven press as mentioned above is used, it is possible to perform working without generating noises. However, a conventional electrically-driven press poses problems as described below. Because the pressing force applied to the workpiece W is determined by the capacity of the drive motor 55, large-capacity pressing machines require the drive motor 55 having a large capacity. Furthermore, in a large-capacity and large-size pressing machine, moving parts including the ram shaft 48 and pressing element 50 also become large both in size and weight. As a result, the driving energy necessary for the repeated vertical movements of the moving parts also becomes large, adding momentum to the undesirable trend toward larger size design and larger capacity design of drive motor 55.
Furthermore, it is difficult to precisely position the pressing element 50 in a prescribed position (height h), for example, above the table 42, and positioning errors frequently occur. Since the pressing element 50 is caused to move vertically by the movement of the nut 49 engaging with the screw shaft 47 as the screw shaft 47 is rotated, it is necessary to increase the number of revolutions and/or the screw pitch of the screw shaft 47 in order to shorten the working cycle time. This results in a decrease in the positioning accuracy of the pressing body 50. On the other hand, reducing the number of revolutions and/or the screw pitch of the screw shaft 47 in order to improve the locating accuracy of the pressing element 50 could increase the time required for the vertical movement of the pressing element 50 and therefore the working cycle time accordingly, resulting in a decrease in working efficiency.
Although there can be another arrangement where the vertical movement of the pressing element 50 is accomplished by use of a plurality of drive means, this requires a complicated structure and a large-sized unit, and it is difficult to smoothly perform the control of a plurality of drive means. Therefore, this method has not been put to practical use.
This invention is intended to overcome the aforementioned problems inherent in the prior art, and it is therefore an object of the present invention to provide a pressure device for press working having high working accuracy and a large pressing force and requiring small driving energy.
To solve the above problems, the present invention adopts a technical means that comprises: a base plate; a support plate spaced at a predetermined distance from the base plate; a first slider and a second slider, both being formed so that they can move between the base plate and the support plate in a direction orthogonal to the base plate and support plate and are capable of relative movement with each other in the above direction, a position sensor for detecting the moving position of the second slider; a first drive means for driving the first slider; a second drive means for driving the second slider; and a central processing unit which controls the first drive means and second drive means and receives and processes position signals from the position sensor. In this technical means, a workpiece placed between the second slider and the base plate is pressed by moving the first slider and second slider to prescribed positions by use of the first drive means and by moving the second slider to a prescribed position by use of the second drive means. Incidentally, the above drive means can include a known speed reduction mechanism having a plurality of gear groups.
In the present invention, the base plate and support plate can be disposed parallel to the horizontal plane and the first slider and second slider can be disposed so that they can move in a vertical direction.
Next, in the present invention, the first drive means may be formed as a crank mechanism and the second drive means as a mechanism comprising a screw pair.
Furthermore, in the present invention, the first drive means and second drive means each can be formed as a mechanism comprising a screw pair.
In this case, the screw in the first drive means can be formed as a ball screw.
Furthermore, in the present invention, the first slider and second slider can be formed so that the relationship between the amount of movement, m1, of the first slider per unit of time and the amount of movement, m2, of the second slider per unit of time is expressed by m1 greater than m2.
Furthermore, in the present invention, motors in the first drive means and second drive means can be formed as servo motors.